Here is the full list of all 8 Zombotron achievements. It takes around 6-8 hours to unlock all of the achievements on Windows.
Defeat the Feral Zombie.
Defeat the Giant Spider.
Defeat the Infator.
Defeat the Stone Golem.
Defeat the Machine Of War.
Land 100 headshots.
Eliminate 500 any enemies.
Find 30 secrets.